A DC-to-DC switched mode power supply may employ a circuit involving an inductor and four switches. A first of the switches (S1) is coupled between a source of a rough DC voltage VIN and a first terminal of the inductor. A second of the switches (S2) is coupled between the first terminal of the inductor and a ground node. A third of the switches (S3) is coupled between a second terminal of the inductor and the ground node. A fourth of the switches (S4) is coupled between the second terminal of the inductor and a load. If the switches are made to switch in a first fashion, then the power supply operates as a “buck converter” in that it converts a higher input voltage VIN into a smaller DC output voltage VOUT that is supplied to the load. If the switches are made to switch in a second fashion, then the power supply operates as a “boost converter” in that it converts a lower input voltage VIN into a larger DC output voltage VOUT on the load.
In some applications, it is useful to switch the switches of the circuit such that the circuit operates as a buck converter at some times and operates as a boost converter at other times. Consider, for example, an application in which a battery is used to supply a circuit with a fixed DC supply voltage. When the battery is fully charged, the DC voltage from the battery is larger than the DC voltage required by the circuit. The intervening power supply therefore operates as a buck converter, receives the higher input battery voltage, and outputs the lower DC voltage required by the circuit. As the battery discharges, however, the DC voltage output by the battery decreases. At a certain point, the DC voltage output by the battery is lower than the DC voltage required by the circuit. The intervening power supply then is made to operate as a boost converter. The boost converter receives the lower DC voltage from the battery and outputs the higher required DC voltage to the circuit. The intervening power supply may be referred to as a “buck-boost” converter. There are many known topologies for buck-boost converters.
FIG. 1 (Prior Art) is a functional block diagram of the TPS63011 average current mode DC-to-DC converter available from Texas Instruments Incorporated of Dallas, Tex. In a buck mode, one of four switches is active, another switch is operating as a rectifier, another switch is permanently on, and the last switch is permanently off. In a boost mode, one switch is active, one switch is operating as a rectifier, one switch is permanently on, and one switch is permanently off. There is no mode of operation in which all four switches are switching. The converter automatically switches from step down operation to step up operation and back as required. Controlling the four switches in this way allows the converter to maintain high efficiency including during times when the input voltage VIN is close to the output voltage VOUT.
FIG. 2 (Prior Art) is a functional block diagram of the LTC3440 buck-boost DC/DC converter available from Linear Technology Corporation of Milpitas, Calif. If VIN is approximately VOUT, then the converter operates in a four-switch buck/boost region of operation. In this mode, the duty cycle of switch SWA is not equal to the duty cycle of switch SWC. There is an amount of time during which SWB and SWC are both off, and SWA and SWD are both on. As disclosed in U.S. Pat. No. 6,166,527, voltage mode control is used to adjust the duty cycle of the switches to maintain VOUT at the desired regulated voltage. If the input voltage VIN is larger than the output voltage VOUT, then SWD is always on and SWC is always off such that the converter operates as a buck converter. If VIN is less than VOUT, then SWA is always on and SWB is always off such that the converter operates as a boost converter.
Although the circuits of FIGS. 1 and 2 operate satisfactorily in many applications, improvements are desired.